Abstract
During the Fronts and Atlantic Storm-Track EXperiment (FASTEX) two large-scale cyclones (in intensive observation periods ii and 17) developed in the North Atlantic. These can be classified, in their initial phase, as Petterssen Type B cyclones. Initiation of the cyclones follows transient upper-level barotropic growth, caused by a pre-existing upper-level trough approaching a jet associated with a strong baroclinic zone. The cyclones later developed further due to baroclinic energy conversion. A favourable orientation of the trough relative to the baroclinic zone ('horizontal tilt against the shear') is crucial for this transient barotropic growth. This scenario is therefore proposed as a general dynamical mechanism leading to Type B cyclogenesis.